1. Field of the Invention
The present invention relates to a photodiode array and image pickup device in which a high-resolution image can be obtained, and a manufacturing method for the same.
2. Description of the Related Art
With the increasing interest in image sensors (image pickup devices) for near-infrared light, a problem of image irregularities caused by cross-talk of light between photodiodes constituting pixels of the image sensors has been a matter of concern (Masatoshi Ishihara, et al., “InGaAs Linear Image Sensor for Near-Infrared Range and their applications”, Technical report of The Institute of Electrical Engineers of Japan, Technical Committee on Light Application and Visual Science, JN: Z0953A Vol. LAV-00 Nos. 7-13; pp. 31-36; (Oct. 26, 2000)). Cross-talk is a phenomenon of degradation of resolution caused by light absorption in adjacent photodiodes, thereby generating a signal current in the adjacent photodiodes (pixels), and a solution to this problem has been desired.
In addition, incident light includes not only near-infrared light but also light of a shorter wavelength range (at the visible light side) in most cases. Accordingly, as shown in FIG. 12, in an image sensor having an epi-side-down mounting structure (substrate-illuminated structure) in which an InP substrate 101 is disposed on the light-incident side, light is absorbed in the InP substrate 101. In this case, a photocurrent diffuses in the InP substrate 101 and, the photocurrent is provided to two or more photodiodes 110. Consequently, an image is blurred, resulting in a problem of degradation of the image quality.
In FIG. 12, a semiconductor multilayer structure includes the InP substrate 101, an InGaAs absorption layer 103, and an InP window layer 104. A p-type region 106 reaching the absorption layer 103 is formed through the InP window layer 104, and a p-n junction 115 is formed in the absorption layer 103. The p-type region 106 is formed by selectively diffusing zinc (Zn) through an opening of a selective diffusion mask pattern 105, and the selective diffusion mask pattern 105 is left as is. The selective diffusion mask pattern 105 is further covered with an anti-reflection (AR) film 109. A p-side electrode 111 is provided so as to form ohmic contact with the corresponding p-type region 106. An n-side electrode 112 that forms a pair with the p-side electrode 111 is also provided. The n-side electrode 112 is electrically connected to a ground potential common to the photodiodes 110. The p-side electrodes 111 function as pixel electrodes and are individually connected to electrode pads 131 of a complementary metal-oxide semiconductor (CMOS) 130. The n-side electrode 112 is connected to a ground electrode pad 132 of the CMOS 130.
In the above image sensor, as the interval between the arrayed photodiodes decreases, degradation of the resolution becomes increasingly significant. Therefore, it is impossible to increase the number of pixels in a sensor portion having certain dimensions. Accordingly, an increase in the chip size required for increasing the number of pixels results in an increase in the cost, and such an increase in the chip size is counter to a reduction in size. To overcome this problem, Japanese Unexamined Patent Application Publication No. 2001-144278 has proposed a photodiode array which has a mesa structure and in which isolation trenches are formed between photodiodes by etching. In addition, Japanese Unexamined Patent Application Publication No. 2005-123217 has proposed a structure in which isolation trenches are provided between photodiodes and a metal film is provided in the isolation trenches, thus isolating the photodiodes with the metal film.
A certain advantage is achieved by the above-described structures in which only isolation trenches or isolation trenches and a metal film are provided between photodiodes. However, light can freely propagate through a semiconductor substrate, and thus light that undergoes multiple reflections through the semiconductor substrate enters adjacent photodiodes. Accordingly, a decrease in the resolution due to cross-talk or the like cannot be sufficiently suppressed.